In the past, these devices have been connected to the public switched telephone network by obtaining a telephone line from the phone company and providing the site to be monitored with a telephone dialler device which is activated by the alarm condition signalled by the above-noted detectors. These prior art monitoring systems communicate with a central site which displays the alarm condition signalled by the monitored site and the operator of the central site contacts the site owners or appropriate civic authority (ie police, fire department etc.) to deal with the alarm condition. Such a scheme requires the installation of an additional telephone line to provide for the service to be carried without interfering with any existing telephone service which the subscriber may have.
The present invention seeks to provide an improved telemetry communications network that does not require a separate telephone line to be operated and can be operated continuously over an existing telephone line without interfering with any use of that line. That is the telemetry communications network will operate irrespective of whether a telephone call is being initiated by rotary or dual tone multi frequency (DTMF) signalling, or is being carried on, or is terminated.